FIG. 1 shows schematic cross-sectional view of a high-k metal gate stack 1 before a method according to an embodiment of the invention is applied. On the bulk silicon 10 of a silicon wafer a number of layers are deposited in this order:
TABLE 1referencenumbermaterialthickness20hafnium oxide as high-k material1-5nm30lanthanum oxide as cap-layer0.2-2nm40Titanium nitride as metal-layer2-50nm50polycrystalline silicon as silicon layer20-100nm60silicon nitride as hard mask60nm
Before depositing the high-k material an interfacial layer ((silicon oxide or silicon oxynitride)) can be optionally deposited at a thickness of up to 1 nm.
Alternatively to the hafnium oxide 20 other materials with a dielectric constant k of greater than 10 (k>10) can be deposited. Suitable materials are e.g. hafnium silicates, zirconium oxides, hafnium silicon oxy nitrides, zirconium silicates, hafnium aluminates, zirconium aluminates, or combinations thereof.
Alternatively to the lanthanum oxide 30 other cap layer materials can be used such as a lanthanide oxide (such as dysprosium oxide).
Alternatively to the titanium nitride as a metal layer other titanium-based or tantalum-based materials or other materials can be used.
Alternatively to the polycrystalline silicon other silicon layers can be used such as amorphous silicon.
Alternatively to the silicon nitride as a hard mask silicon oxide can be used.
Examples for such stacks are described in S. Kubicek et al, IEDM Tech. Dig., p. 49, 2007 and A. Toriumi et al, IEDM Tech. Dig., p. 53, 2007.
A photolithography step is carried out to expose the stack where the stack layers shall be removed in order to expose the bulk silicon. The to-be-removed areas are treated with a plasma process wherein boron chloride and argon are supplied to the plasma chamber. In the to-be-removed areas (where no photo-resist is present) the silicon nitride layer 60, the polycrystalline silicon layer 50 and the titanium nitride layer 40 are generally removed. The lanthanum oxide layer 30 and the high-k layer 20 are modified by the plasma treatment so that modified lanthanum oxide 25 and modified high-k material 35 is generated (see FIG. 1). During the plasma treatment residues are generated. The carbon-rich residues 75 (deriving from photo resist) remain on top of the hard mask 60. Sidewall residues remain on the sidewall of the etched stack, which are basically metal-enriched residues 45 adhering on the sidewall and silicon-enriched residues 55 adhering on the metal-enriched residues.
It is an object of the invention to provide an easy controllable process to selectively remove lanthanum oxide of lanthanide oxide, on which no metal layer 40 or silicon layer 50 remains, and leave a clean structure without undercut of high-k or metal layers. Any attack of other materials such as hafnium or zirconium containing oxides (as high-k materials) and materials exposed at other areas of the structure such as aluminium oxide shall be prevented as far as possible.